As described in co-pending provisional patent application Ser. No. 60/233,870 filed on Sep. 20, 2000 by Rachel Learned and Joseph Su, and incorporated herein by reference, a system is provided for separating interfering signals which utilizes a parameter estimation unit. The purpose of the parameter estimation unit is to be able to derive channel parameters which uniquely distinguish the characteristics of each individual signal regardless of the fact that the signals exist in the same communications bandwidth and at the same instant in time. These parameters are required by any signal separation system for highly loaded or overloaded systems of users and in general include for each signal the channel transfer function comprised of the received power, the phase of the oscillator which generated each received signal, the timing offset relative to the base station clock, any frequency offset of the carrier, and the structure of the multipath replicas.
In reality, however, the received power of the signals vary substantially from burst to burst, which means that the parameter estimation becomes somewhat difficult. Likewise, the phase of the oscillator can also vary from burst to burst, as can the timing offset which is the variance of when the signals are to be received in a particular timing slot of the communications system. Not only can all of these parameters change on a burst by burst basis, the frequency of the carrier can also change, most notably due to Doppler shifts.
The result is that signal separation becomes increasingly difficult in the changing environment where a number of mobile users are trying to communicate with a base station on the same channel.
While the parameter estimation method described in the aforementioned patent application is useful in signal separation, one of its features is that it requires a serial approach to generation of parameter estimation values which overlooks the dynamic nature of the communications environment. In reality, while such a system is able to extract the various parameters of the simultaneously-occurring signals in a quite robust fashion, there remains nonetheless certain times when due to the serial nature of the calculation, there are blanks or blackout periods at which time some of the simultaneously-occurring signals are being ignored or are not being tracked.
For multi-user detection systems, there is therefore a need to be able to dynamically adapt to the changing signals occasioned by the fact that the mobile users are in fact mobile and to the fact that the power adjustment for each of the mobile terminals is adjusted by the base station, sometimes on a burst by burst basis. It will be appreciated that in many multi-user configurations such as with air interfaces including GSM, other TDMA systems and CDMA systems, power level adjustments are made as often as 800 times a second. The result is that a parameter estimation at one particular point in time is invalid if for no other reason than the power level is changed at the handset. While this is done in an organized and routine manner, when considering overlapping or interfering signals the change in power to say nothing of the other parameters, results in an inability to separate interfering signals on the same channel.